In many wireless cellular telecommunications systems such as, for example, UTRAN, (Universal Terrestrial Radio Access Network), GSM (Global System for Mobile Communication, EDGE, (Enhanced Data Rates for Global Evolution, and LTE (Long Term Evolution) networks, user terminals can assume a state which is sometimes referred to as “idle mode”, i.e. a state in which a terminal is dormant or inactive.
In order to ensure that user terminals which are in the idle mode are camped on the correct cell regardless of mobility and variations in radio conditions, there exists a procedure known as cell reselection, by means of which a terminal which is in idle mode may choose a different cell than its present one. The cell reselection procedure which is thus used by user terminals in idle mode can roughly be said to correspond to the handover procedure that is used by the terminals in the active mode. However, for idle mode user terminals, the system or network does not have a control channel by means of which it can order individual user terminals to execute cell reselection.
In many systems, parameters related to cell reselection are transmitted by the base stations of the various cells in the system. The parameters which are broadcast can include, for example, lists of cells that the terminals may reselect, minimum threshold required in order to reselect a particular cell etc.
The idle mode terminals perform downlink measurements on reference symbols such as, in UTRAN, CPICH (Common Pilot Indication Channel) from the different cells that are included in the broadcast list, both regarding received signal strength and signal quality, i.e. interference on the channel. The cycle or intervals with which the terminals make these measurements may also be one of the parameters which are broadcast by the base stations of the cells.
Based on, inter alia, these measurements, the terminals may either reselect a new cell or remain camped on its present cell. If a new cell is reselected, the terminal will, in some systems, send a cell update message to the network in order to allow it to keep track of the terminal's location. This is done in order to ensure that the terminal correctly receives paging and that it will be able to initiate a call without unnecessary signal degradation.
As can be realized from the explanation above, inappropriate cell reselection will increase the call blocking probability upon idle-to-active transition of a user terminal, thereby degrading the service performance of the system. Thus, proper cell reselection is an essential and important feature in cellular systems.
In addition, in many locations, such as, for example so called “hot spots”, an operator may deploy more than one cell in the same geographical area, so that there will be more than one cell in one sector, with the cells having at least a partial overlap. Such cells are also sometimes referred to as co-located cells, a term which will be used in this text. The term ‘co-located cells’ is here used as a generic term, and should thus be understood as referring to both so called inter frequency cells, i.e. cells which are co-located on one and the same base station but with different operating frequencies, as well as cells which are co-located by means of belonging to different systems, such as, for example, UTRAN and GERAN.
Another factor that makes proper cell reselection for idle mode terminals an important feature in a wireless cellular telecommunications system is that it is often undesirable to be forced to make a handover of a terminal to another cell immediately when a terminal has gone from idle mode to an active state.